21021997 has 2 divisors, whose sum is σ = 21021998. Its totient is φ = 21021996.

The previous prime is 21021989. The next prime is 21022007. The reversal of 21021997 is 79912012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20967241 + 54756 = 4579^2 + 234^2 .

It is a cyclic number.

It is not a de Polignac number, because 21021997 - 2³ = 21021989 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21021697) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10510998 + 10510999.

It is an arithmetic number, because the mean of its divisors is an integer number (10510999).

Almost surely, 2^21021997 is an apocalyptic number.

It is an amenable number.

21021997 is a deficient number, since it is larger than the sum of its proper divisors (1).

21021997 is an equidigital number, since it uses as much as digits as its factorization.

21021997 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2268, while the sum is 31.

The square root of 21021997 is about 4584.9751362467. The cubic root of 21021997 is about 275.9887142693.

The spelling of 21021997 in words is "twenty-one million, twenty-one thousand, nine hundred ninety-seven".